Add Merge database utility function (#47)

Thank you to @TheZ3ro and @monomon for there major contributions to this PR!

src/core/Database.cpp

@@ -282,6 +282,12 @@ void Database::recycleGroup(Group* group)
      }
 }
 
+void Database::merge(const Database* other)
+{
+    m_rootGroup->merge(other->rootGroup());
+    Q_EMIT modified();
+}
+
 void Database::setEmitModified(bool value)
 {
     if (m_emitModified && !value) {